This invention relates generally to migration imaging, and more specifically to an overcoated migration imaging member and the process for preparing the member.
Migration imaging systems capable of producing high quality images of high density, continuous tone and high resolution, have been developed. Such migration imaging systems are disclosed, for example, in U.S. Pat. No. 3,909,262 which issued Sept. 30, 1975, the disclosure of which is incorporated herein in its entirety. In a typical embodiment of migration imaging systems, an imaging member comprising a substrate, a layer of softenable material, and photosensitive marking material is imaged by first forming a latent image by electrically charging the member and exposing the charged member to a pattern of activating electromagnetic radiation such as light. Where the photosensitive marking material was originally in the form of a fracturable layer contiguous the upper surface of the softenable layer, the marking particles in the exposed area of the member migrate toward the substrate when the member is developed by softening the softenable layer.
The expression "softenable" as used herein is intended to mean any material which can be rendered more permeable thereby enabling particles to migrate through its bulk. Conventionally, changing the permeability of such material or reducing its resistance to migration of migration marking material is accomplished by dissolving, melting, and softening, by techniques, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means.
The expression "fracturable" layer or material as used herein, means any layer or material which is capable of breaking up during development, thereby permitting portions of said layer to migrate toward the substrate or to be otherwise removed. The fracturable layer may be particulate, semi-continuous, or microscopically discontinuous in various embodiments of the migration imaging members of the present invention. Such fracturable layers of marking material are typically contiguous to the surface of the softenable layer spaced apart from the substrate, and such fracturable layers may be substantially embedded in the softenable layer in various embodiments of the imaging members of the inventive system.
The expression "contiguous" as used herein is intended to mean in actual contact; touching; also, near, though not in contact; and adjoining, and is intended to generically describe the relationship of the fracturable layer of marking material in the softenable layer, vis-a-vis, the surface of the softenable layer spaced apart from the substrate.
There are various other systems for forming such images, where non-photosensitive or inert marking materials are arranged in the aforementioned fracturable layers, or dispersed throughout the softenable layer, as described in the aforementioned patent, which also discloses a variety of methods which may be used to form latent images upon migration imaging members.
Various means for developing the latent images in the novel migration imaging system may be used. These development methods include solvent wash-away, solvent vapor softening, heat softening, and combinations of these methods, as well as any other method which changes the resistance of the softenable material to the migration of particulate marking material through the softenable layer to allow imagewise migration of the particles toward the substrate. In the solvent wash-away or meniscus development method, the migration marking material migrates in imagewise configuration toward the substrate through the softenable layer, which is softened and dissolved, leaving an image of migrated particles corresponding to the desired image pattern on the substrate, with the material of the softenable layer substantially or partially completely washed away. Various methods and materials and combinations thereof have previously been used to fix such unfixed migration images. In the heat, or vapor softening developing modes, the softenable layer is softened to allow imagewise migration of marking material toward the substrate and the developed image member generally comprises the substrate having migrated marking particles nearer the softenable layer substrate interface with the softenable layer and unmigrated marking materials intact on the substrate in substantially their original condition.
The background portions of an imaged member may be transparentized by means of an agglomeration effect. In this system, an imaging member comprising a softenable layer containing a fracturable layer of electrically photosensitive migration marking material is imaged in one process mode by electrostatically charging the member, exposing the member to an imagewise pattern of activating electromagnetic radiation, and the softenable layer softened by exposure for a few seconds to a solvent vapor thereby causing a selective migration of the migration material in the softenable layer in the areas which were previously exposed to the activating radiation. The vapor developed image is then subjected to a heating step causing the migration material in unexposed areas to agglomerate or flocculate, often accompanied by fusion of the marking material particles, thereby resulting in a very low background image. Alternatively, the migration image may be formed by heat followed by exposure to solvent vapors and a second heating step which results in background reduction. In this imaging system as well as in the previously described heat or vapor development techniques, the softenable layer remains substantially intact after development, with the image being self-fixed because the marking material particles are trapped within the softenable layer.
Generally, the softenable layer of migration imaging members is characterized by sensitivity to abrasion and foreign contaminants. Since a fracturable layer is located at or close to the surface of the softenable layer, abrasion can readily remove some of the fracturable layer and adversely affect the final image. Foreign contamination such as finger prints can also cause defects to appear in any final image. Moreover, the softenable layer tends to cause blocking of migration imaging members when multiple members are stacked or when the migration imaging material is wound into rolls for storage or transportation. Blocking is the adhesion of adjacent objects to each other.
The sensitivity to abrasion and foreign contaminants can be reduced by forming an overcoating such as the overcoatings described in the aforementioned U.S. Pat. No. 3,909,262. However, because the migration imaging mechanisms depend critically on the electrical properties of the surface of the softenable layer and on the complex interplay of the various electrical processes involving charge injection from the surface, charge transport through the softenable layer, charge capture by the photosensitive particles and charge ejection from the photosensitive particles etc., application of an overcoat to the softenable layer often causes changes in the delicate balance of these processes, and results in degraded photographic characteristics compared with the non-overcoated migration imaging member. Notably, the photographic contrast density is degraded. Contrast density is the difference between maximum optical density and minimum optical density of an image. Optical density is measured by diffuse densitometers with a blue Wratten No. 94 filter. The expression "optical density" as used herein is intended to mean "transmission optical density" and is represented by the formula: EQU log.sub.10 [l.sub.o /l]
where l is the transmitted light intensity and l.sub.o is the incident light intensity. Using the high density film described in copending application D/82122, entitled "MULTISTAGE DEPOSITION PROCESS", filed in April 1983, in the names of Philip H. Soden and Paul S. Vincett, the entire disclosure of which is incorporated herein by reference, it has been found that the photographic characteristics and particularly the contrast density of the migration imaging member overcoated with the materials and prepared in accordance with the teaching described in the aforementioned U.S. Pat. No. 3,909,262 were greatly degraded when heat-developed. Recent experimental studies of the imaging mechanisms have been conducted by the technique of Thermally Stimulated Current (TSC). The technique of Thermally Stimulated Current is described, for example, in "Thermally Stimulated Discharge of Polymer Electrets" PhD. thesis, University of Leiden, 1972 and "Electrets, Charge Storage and Transport in Dielectrics", edited by M. M. Perlman, 1972, The Electrochemical Society, Inc. These Thermally Stimulated Current experimental studies in both the non-overcoated and overcoated migration imaging members have indicated that the loss of contrast density is due to trapping of the injected surface charge at the overcoat/softenable layer interface. Thus, during heat development, the migration imaging member is subject to the combined effects of a high field and a high temperature, which cause excessive thermally-activated conduction within the unexposed particles similar to the photoconductive process in the exposed particles. As a result, the discrimination (contrast density) between the light-struck and the dark regions is degraded. Moreover, many overcoats do not provide sufficient protection from abrasion and fingerprint contamination.
In addition, many overcoatings do not prevent blocking when migration imaging members are stacked or wound into rolls. In addition, for applications where migration imaging members are utilized for composing printing masters wherein imaged migration imaging members are temporarily secured by adhesive tape to a substrate and thereafter reused, very often the migration imaging member is damaged by removal of the adhesive tape and is rendered unsuitable for reuse. This damage generally takes two forms. First, many overcoats do not adhere well to the softenable layer of the migration imaging member and can be separated by flexing or easily separated or removed entirely from the softenable layer upon removal of the adhesive tape, thereby eliminating further abrasion resistance. Secondly, the softenable layer which contains the photoactive particles often separates from the conductive layer upon removal of the adhesive tape. Therefore, the overcoat should not only adhere well to the softenable layer but should also have abhesive properties to release the adhesive tape to prevent damage to the migration imaging member.
Also, it is a known fact that the charge life, i.e., the permissible time delay between charging and exposure before unacceptable degradation of sensitometric properties occurs, of non-overcoated migration imaging members is only about a few minutes for heat development. This is caused by the rapid dark decay of deposited negative corona charge on the surface of the softenable layer. Yet for many practical applications, it is necessary to extend the charge life of the migration imaging member.
While some of the above-described migration imaging members exhibit certain desirable properties such as resistance to abrasion and foreign contaminants, there continues to be a need for improved migration imaging members. Additionally, there is a need for improved migration imaging members which exhibit greater resistance to the adverse effects of finger prints, blocking, softenable layer/overcoating layer interface failure, and abrasion, can survive adhesive tape tests, and can be vapor or heat developed to provide essentially full contrast density.